feat: batch predict code and text generation

[BigBlackWolf]

Description

As a base sample was taken GoogleCloudPlatform/python-docs-samples#12543

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• vertexai/batch-predict/batch_code_predict.go:17, tag generativeaionvertexai_batch_code_predict
• vertexai/batch-predict/batch_text_predict.go:17, tag generativeaionvertexai_batch_text_predict

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[telpirion]

issue: does this client need a 'defer client.Close()'?